This invention relates to phase separated block copolymers useful as a polymer electrolyte in a battery.
Rechargeable batteries enjoy an enormous and constantly growing global market due to the increased use of portable consumer electronic products. The lithium solid polymer electrolyte battery is an attractive rechargeable battery technology due to its high predicted energy density and low associated materials and processing costs. A successful lithium battery requires the use of an electrolyte that is highly conductive in order to sustain a high density.
Contemporary rechargeable lithium ion batteries utilize a liquid electrolyte and are assembled with a physical separator layer between the anode and the cathode to prevent electrical shorting. The use of a physical separator results in increased costs, due to associated materials and processing. In addition, contemporary liquid electrolytes are volatile at elevated temperatures, exhibit electrochemical breakdown at voltages (xcx9c4.5V) that fail to fully exhibit cathode capacity, and can react chemically with electrode components. This limits both the (total available charge) and the maximum current density and reduces the useful cycle life (number of charge/discharge cycles to failure).
In order to overcome the limitations inherent in liquid electrolytes, solid polymer electrolytes have been developed in which ion mobility is possible through coordination of the lithium ion with suitable sites on the polymer chain. An inherent inverse relationship between ionic conductivity and dimensional stability exists in most known polymer electrolytes. Prior art electrolytes typically demonstrate either good ionic conductivity or good dimensional stability, but not both. Dimensional stability can be achieved by crosslinking, glassification, and the like, but these arrangements generally impede ionic conductivity since conductivity requires a significant degree of polymer chain mobility.
For example, high molecular weight polyethers such as polyethylene oxide (PEO) have been used as lithium salt electrolytes. However, PEO is crystalline at room temperature which as an adverse effect on the conductivity of the polymer. Above the melting point of PEO (Tm=60xc2x0 C.) ionic conductivity increases significantly, but at these temperatures PEO behaves as a viscous liquid and loses much of its dimensional stability (resistance to flow).
Prior efforts have been directed at decreasing the crystallinity of PEO in its solid state through addition of plasticizers or modification of the polymer architecture through random copolymerization and the like. However, these strategies have generally yielded materials with poor mechanical properties, since these materials behave more like liquids than solids.
Crosslinking also has been used as a technique for increasing mechanical rigidity to polymeric electrolytes. A common approach is to prepare network-type structures via irradiation or chemical crosslinking. Not surprisingly, the ionic conductivity of these systems is compromised because the crosslinks limit chain mobility. Furthermore, crosslinked networks of solid polymer electrolyte materials do not flow and are insoluble. Therefore preparation of the electrolyte and its incorporation into and arrangement in the battery may be difficult.
Block copolymers have been proposed for use as solid polymer electrolytes. See, WO 98/16960 by Massachusetts Institute of Technology. Block copolymers are composed of macromolecular moieties (or blocks) of two distinct chemical species that are covalently linked. The chemical connectivity of the blocks results in unique thermodynamic and rheological behavior. At high temperatures or in a common solvent, block copolymers form homogenous phases in which the different blocks are segmentally mixed. Upon lowering the temperature or concentrating the polymer by solvent evaporation, the repulsion between unlike segments increases causing the copolymer to phase separate locally into domains rich in one or the other of the block components. These domains form ordered nanostructures, the morphology of which is governed by the relative volume fraction of the two blocks. The microphase separation process imparts dimensional stability to the material, even for materials in which both blocks individually are inherently amorphous and at temperatures exceeding the glass transition temperature of both blocks.
Published International Application WO 98/16960 describes a solid polymer electrolyte block copolymer that includes an ionically conductive polymer. A continuous lithium ion conducting pathway was obtained. However, the practical cell current is undesirably low. This is attributed to the low transference number for lithium ions in this system (tLI≈0.3-0.5). Stated differently, anionic migration during use may result in a salt concentration gradient in the electrolyte. Anions are attracted to the positive electrode (cathode) causing salt depletion from the electrolyte interior. Such a concentration gradient impedes the movement of the lithium ions between the electrodes, resulting in prolonged and undesirable polarization of the cell.
Significant efforts have been directed toward viable solid polymer electrolytes, yet improvements are greatly needed.
It is an object of the present invention to provide an electrolyte for use in batteries that exhibits good ionic conductivity while retaining good dimensional stability.
It is a further object of the invention to provide an electrolyte that possesses a high transference value for the conductive lithium ion.
It is still a further object of the invention to provide a solid electrolyte in which only the cations have high mobility.
It is still a further object of the invention to provide a solid polymer electrolyte in which the anions are immobilized on the polymer electrolyte.
It is also an object of the invention to provide an electrochemical cell having improved overall cell efficiency due to improvements in the electrolyte.
These and other objects of the invention are achieved in practice of the invention described in the description that follows.
In one aspect, the present invention provides a solid polymer electrolyte including a microphase separated block copolymer comprising at least one ionically conductive block and at least one second block that is immiscible in the ionically conductive block. The solid polymer electrolyte further includes an anion that is immobilized on the polymer electrolyte. The ionically conductive block provides a continuous ionically conductive pathway through the electrolyte. The continuous ionically conductive pathway of the ionically conductive block is due to morphology of microphase separation or defects in microphase separated morphology.
By xe2x80x9cimmobilizedxe2x80x9d as the term is used herein, it is meant that the anion interacts strongly with the copolymer blocks such that there is effectively no migration of the anion at values of potential difference encountered in service in the electrochemical cell into which the electrolyte is introduced. In one preferred embodiment of the invention, the anion is immobilized on the ionically conductive block of the block copolymer. In a second preferred embodiment of the invention, the anion is immobilized on the second block of copolymer.
By xe2x80x9cmicrophase separatedxe2x80x9d as that term is used herein, it is meant that the block copolymer has been subjected to conditions that favor the association of the copolymer chain to form regions or domains within the copolymer containing substantially only a single selected block. The blocks of a microphase separated block copolymer therefore are locally segregated into order domains.
In a preferred embodiment, the microphase separated copolymer blocks are non-glassy and amorphous throughout the temperature range of use, and the mobile cationic species is substantially localized in the ionically conductive block. The cationic species includes lithium, sodium, potassium, magnesium and calcium. In a preferred embodiment of the invention, the anion exhibits delocalized charge density.
The ionically conductive block may include a polymer backbone having polyalkylene oxide or polyalkylene glycol side chains. The polyalkylene oxide or polyalkylene glycol side chains may be of a length less than about 20 oxide units. A preferred polyalkylene oxide is polyethylene oxide. The volume fraction of the ionically conductive block is in the range of about 0.50 to about 0.85.
In one preferred embodiments, the second polymer block is made up of a copolymer comprised of a first monomer selected for its ability to microphase separate from the ionically conductive block and a second monomer comprising an anion or neutral precursor thereof. In other preferred embodiments, the ionically conductive block is made up of a copolymer comprised of a first ionically conductive monomer and a second monomer comprising an anion or neutral precursor thereof. The neutral precursor is convertable into the desired anionic species.
In other preferred embodiments, the polymer electrolyte may further include a conductive liquid. Addition of a conductive liquid results in a ratio of alkylene oxide moiety to lithium ion in the range of 15:1 to 30:1.
In other preferred embodiments, the electrolyte has a transference number of substantially greater than 0.5, preferably greater than 0.8 and more preferably about 0.9 -1.0.
In another aspect, the invention provides a block copolymer including at least one ionically conductive block, at least one second block that is immiscible with the ionically conductive block, an anion immobilized on the polymer electrolyte and a cationic species. The block copolymer may be used as a conductive binder in a cathode or anode.
In another aspect, the invention provides a battery including an electrolyte including a microphase separated block copolymer comprising at least one ionically conductive block and at least one second block that is immiscible in the ionically conductive block. The solid polymer electrolyte further includes an anion that is immobilized on the polymer electrolyte. The ionically conductive block provides a continuous ionically conductive pathway through the electrolyte. The battery further includes a negative electrode in electrical and ionic contact with the electrolyte and a positive electrode separated from the negative electrode in electrical and ionic contact with the electrolyte. An external circuit is in electronic communication with the negative and positive electrodes.
The term xe2x80x9cionic communicationxe2x80x9d is used to indicate a relationship between components of a battery whereby ions are capable of movement or flow with little or no resistance, i.e., under the driving force normally encountered in the operation of a battery. Such a relationship may exist when components are in direct physical contact with each other or when components communicate via intermediate structures which are capable of transporting the ion of interest.
The term xe2x80x9celectronic communicationxe2x80x9d is used to indicate a relationship between components of a battery whereby electrons are capable of movement or flow with little or no resistance, i.e., under the driving force normally encountered in the operation of a battery.